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Error: This assembly may have been downloaded from the Web

If you’re like most programmers you get a LOT of stuff from the Internet, like sample projects, assembly’s (i.e. .DLL’s), etc… It seems like just about everything we do these days in software is somehow related to the Web, even if that means you are just blogging about it.

After upgrading a Silverlight project so that it can be developed on Visual Studio.NET 2010 I ran into a show-stopper error:

Could not load the assembly file:///.... .dll. This assembly may have been downloaded from the Web. If an assembly has been downloaded from the Web, it is flagged by Windows as being a Web file, even if it resides on the local computer. This may prevent it from being used in your project. You can change this designation by changing the file properties. Only unblock assemblies that you trust. See http://go.microsoft.com/fwlink/?LinkId=179545 for more information.

The recommended solution is to “Unblock” the content through Windows Explorer, the only problem is that I was developing this application on a machine at my work and that machine’s Security Settings prevented the “Unblock” feature from Windows Explorer (you couldn’t even see the Unblock button), I thought I was totally screwed.

After banging my head on the desk a few thousand times a co-worker finally suggested that I copy the DLL’s to a Fat32 drive (like an external Thumb drive), then copy them back to the orginal location (over-writing the originals with the copy).

I then simply needed to select “View All Files” for the project in Visual Studio, expand the Bin directory, and delete all the contents (I also removed the reference to the DLL and re-added it just to be safe).

Recompiling the project was successful and I could finally get back to work, what a pain in the a—, hope this helps you fix this issue if you run into it